Extreme ultraviolet (EUV) lithography, which is based upon exposure with the portion of the electromagnetic spectrum having a wavelength of 10-15 nanometers, may be used to print features with smaller critical dimension (CD) than other more conventional techniques, such as those utilizing deep ultraviolet (DUV) radiation. For example, an EUV scanner may use 4 imaging mirrors and a Numerical Aperture (NA) of 0.10 to achieve a CD of 50-70 nm with a depth of focus (DOF) of about 1.00 micrometer (um). Alternatively, an EUV scanner may use 6 imaging mirrors and a NA of 0.25 to print a CD of 20-30 nm although the DOF will be reduced to about 0.2 um.
Masking and reflection of EUV radiation brings about a unique set of challenges generally not encountered with DUV radiation. For example, a mask for DUV lithography is transmissive, and layers of materials such as chrome and quartz may be used to effectively mask or transmit, respectively, DUV radiation. Thus, a desired pattern on a DUV mask may be defined by selectively removing an opaque layer, such as chrome, to uncover portions of an underlying transparent substrate, such as quartz. However, virtually all condensed materials absorb at the EUV wavelength, so a mask for EUV lithography is reflective, and the desired pattern on an EUV mask is defined by selectively removing portions of an absorber layer (“EUV mask absorber”) to uncover portions of an underlying mirror coated on a substrate. The mirror, or reflective multilayer (“ML”), generally comprises a number of alternating layers of materials having dissimilar EUV optical constants or indices of refraction. A cap or capping layer may be positioned upon the reflective multi-layer to protect the multi-layer from degradation during process treatments. For example, a silicon cap layer may be used to prevent oxidation of molybdenum layers comprising a reflective multi-layer.
Selective removal of portions of the EUV mask absorber generally involves etching spaces or trenches through portions of the EUV mask absorber material, and the CD uniformity and bias are highly dependent upon the accuracy of such etching. Toward the end of defect-free mask and print features for making microelectronic devices, inspection and repair techniques are utilized before EUV irradiation. To protect the reflective multi-layer during repair procedures, a buffer layer may be positioned between the reflective multi-layer and the EUV mask absorber material. Subsequent to repair, the buffer layer is etched away in preparation for EUV irradiation of the reflective multi-layer.
The integration of a buffer layer, and subsequent removal of a portion thereof, adds complexity and expense to the EUV mask patterning process. The use of a buffer layer also generally results in a thicker absorber stack, which increases undesirable shadowing effects in stepper imaging. A more simplified integration providing similar functionality would be preferred.